1. Field of the Invention
This invention relates to electrical power generation, and more particularly to a controller designed to connect and disconnect an induction, or asynchronous, generator driven by a random energy source, such as a wind driven turbine, to the electrical utility power mains.
2. Description of the Prior Art
Wind turbine generators are coming into more frequent use as an alternative electrical power source. Wind farms use induction generators to convert the rotary movement of a wind turbine to electrical power.
The fact that the wind velocity is random, unpredictable and subject to rapid changes complicates the manner in which the generator is connected to the AC power mains. The same problem exists for other induction generators whose energy supply is subject to random fluctuations, such as gas or liquid powered generators. Induction machines are inherently capable of operating either as generators or motors, depending on the rotational velocity of the prime mover drive. For velocities greater than the machine's synchronous speed, the machine will operate in a generating mode and provide a power output to the mains. However, for velocities below the synchronous speed, the machine will operate in a motoring mode and draw power from the mains. If adequate controls are not provided, the machine can draw more energy than it delivers under low level fluctuating wind conditions. This has led to the development of various types of switching arrangements that disconnect the machine from the mains at less than synchronous speeds, and reconnect the machine to the mains when its speed reaches synchronous or above.
In an early type of controller for a wind turbine generator, the turbine was held stationery with a brake until the wind speed exceeded a threshold level. When the threshold was exceeded, the brake was released and the generator immediately connected to the main power line with an electro-mechanical contactor. The machine then motored rapidly to synchronous speed, and was driven above synchronous speed by the turbine torque. When the wind speed dropped, a timer held the contactor closed for a pre-determined period of time before allowing the contactor to open and disconnect the generator from the power line. This type of generator control suffers from high in-rush current and starting torque, and also wastes energy by motoring when the wind speed is low.
More advanced controllers, such as those manufactured by the Vestas Corporation, operate with a free-turning propeller and utilize a pulse counting generator shaft speed sensor to signal the correct instant to close the contactor. In these devices the correct determination of the contact closure time is critical, especially if the generator is rapidly accelerating through synchronous speed. The torque-to-inertia ratio of modern wind turbine generators is very high. As a result, acceleration can reach 1,000 rpm/sec. (1 rpm/msec) at high wind velocities. Also, these induction generators are very low slip machines, with rated torque for an 1,800 rpm machine typically occurring at 1,775 rpm during motoring and 1,825 rpm during generating. Thus, at high acceleration, the typical 25 msec time lag for a contactor to close can result in the connection to the main power line being made a 1,800+(1.0 rpm/msec.times.25 msec)=1,825 rpm instead of the desired 1,800 rpm. The result is severe torque transient which shortens the life of the mechanical drive train.
Mains frequency deviation is another source of difficulty with controllers that rely on a shaft speed sensor to determine the instant that the generator is connected to the mains. Thus, for instance, a shift in frequency from 60 Hz to 60.5 Hz changes the synchronous speed of a four pole machine from 1,800 rpm to 1,800+1,800.times.0.5/60=1,815 rpm. Connecting the machine to the mains at 1,800 rpm with speed sensor control will then produce a large in-rush current and torque pulse as the machine motors up to its 1,815 rpm synchronous speed.
To cope with contactor time lag and mains frequency deviation, contemporary wind generator controllers utilize a micro-computer and complex algorithms to energize the contactor at the precise instant of time, based on speed, acceleration and mains frequency to achieve a connection to the mains at a speed as close as possible to synchronous speed.
Other modern free-turning generator controllers use a modified solid-state reduced voltage starter to connect the generator to the mains in response to a tachometer-derived control logic signal. The Westinghouse Electric Corporation 60-63PA Series, described in Operating and Service Manual for 60-63PA Series Phase Proportioning SCR Power Controller, Westinghouse Electric Corp., Oldsmar, Fla., vol. OSM108-May, 1982, is typical of this type of controller, which uses a thyristor switching arrangement between the generator and power line. A thyristor is a semiconductor device that turns on when a momentary pulse of gating current is received, and typically can be turned off only by interrupting its working current. In this type of controller, the thyristor gating is initiated at a relatively large delay angle into each half cycle of the main power line; since the thyristors are turned off when the alternating current passes through zero, the thyristors are closed for only a relatively short portion of each cycle and thereby limit the power transfer between the power line and induction machine to a low value. The thyristor gate delay angle is then ramped down to zero at a pre-determined rate, resulting in the application of an increasing effective voltage to the generator with a controlled in-rush current.
Because of the relatively slow application of voltage with this controller, the thyristor gating must commence at a speed well below synchronous speed (typically 1,750 rpm) to achieve full voltage at a synchronous speed of 1,800 rpm under high acceleration conditions. Because the generator voltage remains fully on after reaching the mains voltage level, the generator will continue to operate as a motor when the wind velocity drops. The resulting motoring torque prevents the generator speeds from falling below the 1,750 rpm cut-in speed. To compensate for this, a complex system of tachometer decoding logic and timers must be used to inhibit the thyristor gating and prevent excessive power consumption when motoring in light winds. Also, a relatively expensive tachometer is necessary for the thyristor-controlled switching scheme, whether the thyristors are turned fully on initially or are ramped up to a fully on state over time.
A further improvement to the thyristor based controller is disclosed in U.S. Pat. No. 4,473,792 to Nola. This patent is directed to a controller for a single phase wind generator. A thyristor is gated on after a fixed firing delay angle late into each half-cycle of the power line. As the generator's speed increases in the vicinity of synchronous speed, the power factor angle inherently increases from less than 90.degree. to more than 90.degree.. By definition, this increases the current lag relative to the line voltage, causing the zero current thyristor cut-off point during each half-cycle to progressively move toward the gate delay point. As a result, the period of time during which the thyristor is on, and thus the effective generator voltage, increases with increasing speed in the vicinity of synchronous speed.
This response of generator voltage to speed in the vicinity of synchronous speed accomplishes two desirable objectives: 1. it effects a connection between the mains and the generator at synchronous speed and above without the use of any speed sensing auxiliary circuitry, and, 2. it reduces the generator losses and increases the generator power output by applying a voltage less than the mains voltage to the generator at low shaft input power levels.
A torsional instability has been observed when the single phase controller concept disclosed in U.S. Pat. No. 4,473,792 is extended to large (50+ kW) three phase wind turbine generators. This instability is caused by a very rapid change in generator voltage with speed in the vicinity of synchronous speed which is a consequence of the very rapid change in generator power factor angle with speed in high power, low slip, induction generators. The rapid change in generator voltage produces a rapid change in generator reaction torque which can excite torsional vibrations in the drive train.
An additional limitation of the control concept of U.S. Pat. No. 4,473,792 is the lack of adequate means to adjust the generator voltage vs. speed profile to optimize the generator power output. It is well known that there is an optimum operating voltage at each induction machine load point that produces maximum machine efficiency. The cited invention discloses only a means for adjusting the low speed excitation voltage. A means of adjusting the rate of change of voltage vs. speed is lacking. Thus the controller cannot be tailored to approximate the optimum voltage vs. speed profile.